A liquid crystal display device includes a pair of substrates and a liquid crystal layer interposed therebetween, and performs displaying by utilizing changes in the alignment direction of liquid crystal molecules based on a voltage which is applied across the liquid crystal layer. Conventionally, the alignment direction (pretilt direction) of liquid crystal molecules while no voltage is applied across the liquid crystal layer is regulated by an alignment film. For example, in a TN-mode liquid crystal display device, pretilt azimuths of liquid crystal molecules are regulated by performing a rubbing treatment for a horizontal alignment film. As used herein, a pretilt azimuth refers to components within the liquid crystal layer plane (within the substrate plane) of a vector which indicates an alignment direction of a liquid crystal molecule in a liquid crystal layer across which no voltage is applied. Note that a pretilt angle, i.e., an angle between an alignment film and a liquid crystal molecule, is primarily determined by the combination of an alignment film and a liquid crystal material. A pretilt direction is expressed in terms of a pretilt azimuth and a pretilt angle.
As a technique for controlling the pretilt direction of liquid crystal molecules, Polymer Sustained Alignment Technology (hereinafter referred to as the “PSA technique”) was developed in the recent years (see Patent Documents 1, 2, and 3). The PSA technique is a technique where a liquid crystal material having a small amount of photopolymerizable compound (which typically is a photopolymerizable monomer) mixed therein is sealed in a liquid crystal panel, and thereafter the photopolymerizable compound is irradiated with light (e.g., ultraviolet) while a predetermined voltage is applied across the liquid crystal layer, so that the generated polymerization product will control the pretilt direction of the liquid crystal molecules. A layer which is formed of the aforementioned polymerization product will be referred to as an alignment sustaining layer in the present specification.
Adopting the PSA technique allows an alignment state of liquid crystal molecules when generating the polymerization product to be maintained (memorized) even after the voltage is removed (i.e., in the absence of an applied voltage). Therefore, the PSA technique has an advantage in that the pretilt azimuths and pretilt angles of liquid crystal molecules can be adjusted by controlling an electric field which is created across the liquid crystal layer, etc. Moreover, since the PSA technique does not require a rubbing treatment, it is particularly suitable for producing a vertical-alignment type liquid crystal layer, whose pretilt direction is difficult to be controlled through a rubbing treatment. Therefore, the PSA technique has so far been vigorously studied for realizing practical applications in combination with vertical alignment films.
However, an alignment sustaining layer (which is a polymerization product of a photopolymerizable compound) used in the PSA technique may be combined with horizontal alignment films. In recent years, horizontal alignment films are used not only in the aforementioned TN-mode liquid crystal display devices, but also in the liquid crystal display devices of lateral electric field modes, e.g., the IPS (In-Plane Switching) mode and the FFS (Fringe Field Switching) mode. By forming an alignment sustaining layer on a horizontal alignment film, an improved alignment regulating force is obtained, and image sticking can be prevented. When forming an alignment sustaining layer on a horizontal alignment film of a liquid crystal display device of a lateral electric field mode, an alignment treatment such as rubbing or light irradiation needs to be performed for the horizontal alignment film in advance. On the other hand, there is no need to apply a voltage across the liquid crystal layer in the step of forming the alignment sustaining layer because, in a lateral electric field mode, the alignment direction of liquid crystal molecules will change within a plane which is parallel to the substrate plane, so that the pretilt angle may almost be zero.
With reference to FIG. 5 and FIG. 6, the effect of image sticking prevention with alignment sustaining layers is described. FIG. 5(a) and FIG. 6(a) are diagrams showing cross-sectional structures of liquid crystal display devices 900A and 900B of the IPS mode (where the electrode structures are omitted). The liquid crystal display devices 900A and 900B each includes a first substrate 910 and a second substrate 920, and a liquid crystal layer 930 interposed therebetween. Moreover, the first substrate 910 and the second substrate 920 of the liquid crystal display devices 900A and 900B have horizontal alignment films 912 and 922 which are provided on the liquid crystal layer 930 side.
However, the liquid crystal display device 900B shown in FIG. 6(a) further includes alignment sustaining layers 940a and 940b between the horizontal alignment films 912 and 922 and the liquid crystal layer 930, whereas the liquid crystal display device 900A shown in FIG. 5(a) includes no such alignment sustaining layers.
FIG. 5(b) and FIG. 6(b) are photographs showing results of conducting an evaluation test concerning the anti-image sticking property of the liquid crystal display devices 900A and 900B shown in FIG. 5(a) and FIG. 6(a). In the evaluation test, first, the display region of each of the liquid crystal display devices 900A and 900B was divided into two regions R1 and R2, and a white voltage (specifically, 6 V) was applied across the liquid crystal layer 930 for a predetermined time (specifically, 6 to 24 hours) in the one region R1 while no such voltage application was performed in the other region R2. FIG. 5(b) and FIG. 6(b) show states where a gray scale voltage for presenting an about 1% luminance (i.e., a voltage corresponding to the 32nd gray scale level when displaying in 256 gray scale levels) of the luminance under a white voltage is applied across the liquid crystal layer 930 in both regions R1 and R2 thereafter.
In the liquid crystal display device 900A having no alignment sustaining layers formed therein, the luminance in the region R1, which received white voltage application, has a higher luminance than does the region R2, which did not receive white voltage application; thus, image sticking has occurred. On the other hand, in the liquid crystal display device 900B having the alignment sustaining layers 940a and 940b formed therein, the luminance in the region R1, which received white voltage application, is essentially equal to that in the region R2, which did not receive white voltage application; thus, image sticking is prevented.